The invention relates a measurement device for measuring a property of a fluid, in particular a concentration of a substance or an ion concentration in the fluid or a pH-value of the fluid, comprising a housing comprising a housing section to be immersed into the fluid during measurement operation and an aperture foreseen in an outside wall of the housing section, in particular in a side wall surrounding an interior of the housing section or in a front wall closing off a front end of the housing section, for exposing a single sensor for measuring the property of the fluid to the fluid, when said housing section is immersed into the fluid.
Measurement devices of this type are frequently used in laboratory measurements technology as well as in industrial process measurements technology in many fields of application, e.g. in chemistry, environmental analysis, biochemistry, biotechnology, pharmacy, food technology and water management.
These measurement devices comprise a single sensor capable of measuring the property required in the specific application, e.g. an electrochemical sensor, e.g. a potentiometric or an amperometric sensor, for determining a concentration of a certain substance or an ion concentration in the fluid or a pH-value of the fluid.
For these measurement devices, it is known in the art, to mount the sensor on an outside surface region of a mechanical support, which is inserted into a housing section of a housing to be immersed into the fluid during measurement and to provide an aperture in an outside wall of the housing section, through which the sensor is exposed to the fluid when the housing section is immersed into the fluid.
Unfortunately sensors, in particular electrochemical sensors, are sensitive elements frequently requiring replacement, re-calibration and/or cleaning. Replacement can e.g. be required due to damage of sensor components like e.g. ion-sensitive membranes, glass domes or glass bulbs of measurement cells of the sensor, occurring due to process conditions or due to operator induced strains the sensor is exposed to. Re-calibration is quite often performed periodically in order to ensure that the sensor is compliant to a measurement accuracy specified for it. It is e.g. applied in order to compensate for drifts of an offset of the sensor and/or changes of the span of the sensor, which may e.g. occur due to aging or wear of components of the sensor exposed to the fluid. Frequent cleaning is e.g. required in applications, where the sensor is likely to become soiled, coated and/or contaminated due to process conditions it is exposed to.
Each time replacement, re-calibration or cleaning is required the mechanical support carrying the sensor has to be taken out of the housing in order to enable the operator to replace the sensor or to transfer it to a treatment site, where it can be cleaned and/or calibrated. This is a time and cost intensive procedure, which quite often requires an interruption of an ongoing process performed at the measurement site.
In U.S. Pat. No. 9,146,138 B2 a measurement device for measuring a property of a fluid is described, comprising:                a housing comprising a housing section to be immersed into the fluid during measurement operation, and        an aperture foreseen in an outside wall of the housing section, in particular in a side wall surrounding an interior of the housing section or in a front wall closing off a front end of the housing section, for exposing a single sensor for measuring the property of the fluid to the fluid, when the housing section is immersed into the fluid.        
The measurement device comprises a single sensor, which is mounted on a movable mechanical support movably secured inside the housing section by means allowing for the mechanical support to be moved into a measurement position, wherein the sensor is exposed to fluid entering the aperture and into a treatment position, wherein the sensor is facing a recess located in an inside wall of the housing. The recess is closed off by an outside surface of the support and is filled with a rinsing and/or calibration fluid. During normal operation, the support is kept in its measurement position. Whenever re-calibration or cleaning is required, the support is transferred into its treatment position, allowing for the sensor to be cleaned or re-calibrated on site. This has the advantage that the support carrying the sensor only needs to be taken out of the housing in case the sensor or the rinsing and/or calibration fluid needs to be replaced.
Nonetheless, the operating time, during which this device can be operated without any need for an ongoing process to be interrupted, is still limited to a time span given by the time between consecutive replacements of the sensor and/or the rinsing and/or calibration fluid.
Whereas re-calibrations of the sensor can be scheduled based on predetermined calibration time intervals, replacements of the sensor may become necessary at unexpected times. In this case the time required to perform the replacement as well as an interruption of an ongoing process required to perform the replacement may cause additional costs and severe inconveniences for the operator.
It is an object of the invention to provide a measurement device, allowing for quick and easy replacement of its sensor.
To this extent, the invention comprises a measurement device for measuring a property of a fluid, in particular a concentration of a substance or an ion concentration in said fluid or a pH-value of said fluid, comprising:                a housing comprising a housing section to be immersed into the fluid during measurement operation, and        an aperture foreseen in an outside wall of the housing section, in particular in a side wall surrounding an interior of the housing section or in a front wall closing off a front end of the housing section, for exposing a single sensor for measuring the property of the fluid to the fluid, when the housing section is immersed into the fluid, wherein according to the invention        a number of least two sensors for measuring the property are foreseen,        each of the sensors is mounted in a different outside surface region of a movable mechanical support, and        said mechanical support is movably secured inside the housing by means allowing for the support to be transferred into a number of different measurement positions, wherein in each measurement position a different outside surface region of the support comprising one of the sensors is exposed to the aperture.        
The invention further comprises a first refinement of the measurement device, wherein each of the sensors is located inside a recess foreseen in the corresponding outer surface region of the support, in particular a recess comprising a depth which is dimensioned such that the sensor does not extend beyond a frontline defined by an outside surface of said support surrounding the recess.
The invention further comprises a second refinement, wherein the means allowing for the support to be transferred into the predefined measurement positions comprise:                means allowing for the support to be transferred manually, in particular means comprising a ratcheting mechanism interlockingly engaging the support every time one of the measurement positions is reached, or        a drive, in particular an electric motor or solenoid, for transferring the support into the predefined measurement positions, in particular a drive transferring the support from one measurement position into a consecutive measurement position each time it is activated, or        a ratcheting mechanism for transferring the support into the predefined measurement positions, in particular a ratcheting mechanism transferring the support from one measurement position into a consecutive measurement position each time it is activated.        
The invention further comprises a third refinement, wherein                said support comprises a cylindrical section, which is rotatably secured inside a cylindrical interior of said housing section by securing means, in particular a retaining ring, allowing for said support to be transferred into said different measurement positions by rotating said cylindrical section around its longitudinal axis, and        either said sensors are arranged in a circle on outer surface regions of a front wall of said cylindrical section resting on said front wall of said housing section, wherein said aperture is foreseen in an off-centered position in said front wall corresponding to the positions of said sensors on said front wall of said support, or        said sensors are arranged in a circle on outer surface regions of a cylindrical wall of said cylindrical section surrounded by said side wall of said housing section, wherein said aperture is foreseen in said side wall at a height corresponding to the height of said sensors on said cylindrical section of said support.        
The invention further comprises a fourth refinement, wherein                said mechanical support comprises a conveyor belt, in particular a conveyor belt comprising two to side sections extending in a direction essentially parallel to a longitudinal axis of said housing section along opposing inside surfaces of said housing section and one or two connecting sections connecting the two side sections,        said sensors are mounted on outside surface regions, in particular in recesses foreseen in said outside surface regions, distributed along a length of said conveyor belt, and        said means allowing for the support to be transferred into said predefined measurement positions comprise conveying means allowing for the support to be transferred into said different measurement positions by conveying the conveyor belt, such that in each measurement position a different sensor located in one of said outside surface regions of said conveyor belt is exposed said aperture.        
According to a first preferred embodiment of the fourth refinement,                said aperture is located in said front wall of said housing section and said conveyor belt comprise an essentially flat region, larger than the size of said sensors, abutting on a flat inside surface of a front wall of said housing section surrounding said aperture, or        said aperture is located in said side wall of said housing section and said conveyor belt comprises an essentially flat region, larger than the size of said sensors, abutting on a flat inside surface of said side wall of said housing section surrounding said aperture.        
According to a second preferred embodiment of the fourth refinement, said conveyor belt is mounted on a mounting frame, in particular a mounting frame equipped with ball bearings allowing for said conveyor belt to be rolled along the mounting frame, in particular a mounting frame supporting outer rims of said conveyor belt.
According to a third preferred embodiment of the fourth refinement,                said conveyor belt forms a closed loop, comprising two to side sections extending in a direction essentially parallel to the longitudinal axis of the housing section along opposing inside surfaces of the housing section and two connecting section connecting the two side sections completing the closed loop, and        said means allowing for the support to be transferred into the different measurement positions comprise:                    an opening in said housing for exposing a section of said conveyor belt allowing for the conveyor belt to be conveyed manually, or            conveying means, in particular one or two toothed wheels comprising teeth successively engaging and disengaging into a section, in particular one of said connecting sections, in particular an outer rim of said connecting section, of said conveyor belt, when said conveyor belt is conveyed forward, allowing for the conveyor belt to be conveyed manually or comprising a drive, in particular a drive turning at least one of said wheels, in particular a drive set up to convey the conveyor belt from one measurement position into a consecutive measurement position each time it is activated.                        
According to a fourth preferred embodiment of the fourth refinement,                said means allowing for the support to be transferred into said different measurement positions comprise a ratcheting mechanism engaging a section of said conveyor belt, in particular a section extending into a housing section adjacent to said housing section to be immersed into said fluid,        in particular a ratcheting mechanism designed to convey said conveyor belt forward by distance corresponding to a distance between consecutive sensors on said outer surface regions of said conveyor belt each time it is activated, in particular a ratcheting mechanism designed to be activated manually, in particular via a lever or a push button, or via an electrical signal.        
According to a fifth preferred embodiment of the fourth refinement, said conveyor belt comprises connecting means foreseen at its opposite ends allowing for the conveyor belt to be connected to a replacement conveyor belt of identical design.
According to a sixth preferred embodiment of the fourth refinement,                said sensors comprise a mechanically rigid base and are mounted directly onto a surface, in particular a surface of a recess foreseen in the respective outside surface regions of the conveyor belt, or        said sensors are mounted on mechanically rigid elements, e.g. ceramic discs, foreseen at the respective outside surface regions of said conveyor belt, or        reinforcements reinforcing the sections of said conveyor belt comprising said sensors are foreseen.        
The invention further comprises preferred embodiments, wherein                said sensors are calibrated sensors, which were calibrated before said measurement device is put into measurement operation, and/or        said sensors are electrochemical sensors, in particular potentiometric or amperometric sensors, in particular electrochemical sensors comprising an analyte sensitive component, in particular membrane or a semiconductor-element, in particular a semiconductor-element comprising an Electrolyte-Insulator-Semiconductor structure, in particular an ion sensitive field effect transistor (ISFET) or a capacitor, whose capacitance depends on the concentration of the substance to be determined.        
The invention further comprises preferred embodiments, wherein                all of said sensors are permanently connected to a measurement electronics via connecting lines, or        each of said sensors comprises a sensor module allowing for said sensor to be powered and for its measurement results to be transferred wirelessly, or        each sensor is connected to a contact and only the sensor exposed to said aperture is connected to a measurement electronics via a contact mounted inside said housing section and connected to said measurement electronics, which is pressed against the contact connected to said sensor, when said support is transferred into the measurement position, wherein said sensor is exposed to said aperture.        
The invention further comprises a fifth refinement, wherein                at least one temperature sensor, in particular a thermo element, for measuring a temperature prevailing at a measurement site is foreseen, in particular a temperature sensor, which is located near said aperture, in particular a temperature sensor, which is mounted independently of the support carrying said sensors, and        said temperature sensor is connected to a measurement electronics designed to perform temperature compensations of temperature dependent measurement errors of measurement results obtained by said sensors.        
The invention further comprises a sixth refinement, wherein sealing means are foreseen, providing a seal between said aperture and an interior of said housing section, in particular sealing means provided by:                at least one surface of said support, in particular a surface of a cylindrical section or surfaces of sections of a conveyor belt of said support, surrounding said sensors, sealing by abutting on an inner surface of said housing section surrounding said aperture, or        a sealing element filling a gap between said support and said housing section and comprising an opening located adjacent to said aperture.        
It is an advantage of the invention, that the sensor performing the measurements can be easily replaced by a new one simply by transferring the support into the next measurement position. This way, the operating time, during which this device can be operated without any need for an ongoing process to be interrupted, is much longer than the time span a single sensor can be operated for before it requires cleaning, recalibration or replacement.
The invention and further advantages are explained in more detail using the figures of the drawing, in which four exemplary embodiments are shown.